1. Field of the Invention
The present invention relates to a fuel assembly used for a nuclear reactor, and relates in particular to a device for systematic vibration prevention.
2. Technical Background
An example of a fuel assembly for use in a pressurized light water reactor is shown in FIG. 9.
In this drawing, the fuel assembly is provided with an upper nozzle 1, and a lower nozzle 2 which is oppositely spaced apart from the upper nozzle 1. A plurality of control rod guide pipes 3 are fixedly disposed between the upper nozzle 1 and the lower nozzle 2, and a plurality of support grids 4 are attached to the control rod guide pipes 3 in the middle section of the fuel assembly.
As shown in FIGS. 10 and 11, the support grid 4 comprises a plurality of straps 7 crossing at right angles to each other to produce a grid network and a plurality of grid cells 5 therein. A plurality of dimples 9 and springs 10 are provided on the opposing walls surrounding the grid cells 5 to provide support to the fuel rods 6 passing through the grid cells 5. The fuel rods 6 are elastically held in the grid cells 5 by being pressed by the springs 10 against the dimples 9.
In a perspective plan view of a grid cell shown in FIG. 11, it is seen that a pair of mixing vanes 11 protrude from the top edge near the intersections of the crossing straps 7. The function of the mixing vanes 11 is to generate a turbulent flow of cooling water for efficient removal of the heat generated by the fuel rod 6.
The mixing vanes 11 are bent inward of the grid cell 5 at the top edge of the strap 7 so that the flow of the cooling water forcibly directed upwards from below (in the drawing) will be disturbed and stirred by the mixing vanes 11. The direction of the bend of the mixing vanes is determined as follows.
As shown in a plan view of the support grid 4 in FIG. 12, a pair of mixing vanes 11 are disposed at opposing corners (lower right and upper left corners in FIG. 12) of each of the gird cells 5, and are bent inwards on the open space of the grid cells 5 throughout an entire fuel assembly.
However, in such a configuration, because the mixing vanes 11 are disposed unidirectionally in a diagonal direction in the support grid 4, the cooling water is extremely vulnerable to cause vibration of the entire fuel assembly due to an unbalanced vibrational attenuation characteristics of each support grid. Furthermore, if vibrations are generated in the fuel rods 6, the fuel rods 6 are subjected to unidirectional vibrational forces (in the direction of arrow a in FIG. 12), thus worsening the resulting systematic vibration of the fuel assembly.